292 THE EVOLUTION THEORY 



about in its present precision by that means, but it is underestimating 

 the power of natural selection not to credit it with being able to adapt 

 a species on one and the same area to different conditions of life, 

 and we shall return to this point later on in a different connexion. 

 But in the meantime it must suffice to point out that the poly- 

 morphism of the social insects affords a proof that a species may 

 break up into several forms in the same area through the operation 

 of natural selection alone. 



I am therefore of opinion, with Darwin and Wallace, that 

 adaptation to new conditions of life has, along with isolation, had 

 a material share in the evolution of the large number of endemic 

 species of snail on the oceanic islands. This brings us to the co- 

 operation of natural selection and isolation. If, thousands of years 

 ago, by one of the rarest chances, an Acliathia-W^Q snail was carried 

 by birds to the Sandwich Islands, it would spread slowly, at first 

 unvaried, from the spot where it arrived over the w^hole of the 

 snailless island. But during this process of diffusion it would fre- 

 quently come in contact with conditions of life which would not 

 prevent it from penetrating further, but to which it was imperfectly 

 adapted, and in such places a process of transformation would begin, 

 which would consist in the fostering of favourably varying individuals, 

 and which would run its course quietly by means of personal 

 selection, based upon the never-ceasing germinal selection, and 

 unhindered by any occasional intrusion of still unvaried members 

 of the species from the original settlement on the island. But these 

 new conditions were not merely different from those of the ancestral 

 country ; the island region itself presented very diverse conditions, 

 to which the snail immigrant had to adapt itself in the course of 

 time, as far as its constitution allowed. Terrestrial snails are almost 

 all limited to quite definite localities with quite definite combinations 

 of conditions ; none of our indigenous species occurs everywhere, 

 but one species frequents the woods, another the fields ; one lives on 

 the mountains, another in the valleys ; one on gneiss soil, another 

 on limy soil, a third on rich humus, a fourth on poor river- 

 sand ; one in clefts and hollows among damp moss, and another 

 in hot, dry banks of loess, and so on. Although we cannot see in 

 the least from the structure of the animal why this or that spot 

 should be the only suitable one for this or that species, we may say 

 with certainty that each species remains permanently in a particular 

 place because its body is most exactly adapted to the conditions 

 of life there, and therefore it remains victorious in the competition 

 with other species in that particular spot. 



